Mario's Tennis

Expert mode:
Press Left Trigger(2), Right Trigger, Left Trigger, Right Trigger, Select at the title screen. A sound will confirm correct code entry. 

More powerful serve:
Press A + B to start the serve. 

Bonus characters:
Successfully complete the game under singles mode using Mario five times, Donkey Kong five times, and Koopa five times. Wario and Rush (Mega Man's companion) will become selectable characters.